This invention relates to a method and apparatus for processing image signals and, more particularly, to a method and apparatus for processing image signals in such a manner that when the image information to be halftoned contains a regular or periodic pattern, the averaging mask size is first determined in accordance with the pitch of said regular or periodic pattern of its combination with the factor of scaling from an input to an output image and said image information is subsequently smoothed (processed for unsharpness) or, alternatively, said image information is first processed for unsharpness and then the portion of the resulting unsharpness signal which has experienced a large change is selectively processed for enhanced sharpness, thereby reproducing a sharp image free from a moire pattern. Then, the averaging mask size is the mask size of averaging the image signals of the pixel of interest and the surrounding pixels for smoothing the image signal of interest.
Image scanning reading and reproducing systems are widely used in the printing and platemaking industries for electrically processing the image information on originals to produce original film plates with a view to simplifying the entire process and improving the quality of printed images.
The image scanning reading and reproducing system is composed basically of an image reading unit and an image recording unit. In the image reading unit, the image information on the original which is transported in an auxiliary scanning direction is scanned with a photosensor in a main scanning direction which is normal to the auxiliary scanning direction, whereby scanned image information is converted to an electric signal. The image information which has been subjected to photoelectric conversion in the image reading unit is then processed in a predetermined manner according to specified platemaking conditions. Thereafter, the processed image signal is converted to a signal of light such as a laser beam and recorded on an image recording medium of a photosensitive material such as a photographic film in the image recording unit. The image recording medium carrying the recorded image is subsequently developed with an image developing device and will be used as an original film plate for printing and other purposes.
In the image scanning reading and reproducing system of the type described above, a continuous-tone image is converted to a halftone image in order to reproduce its gradations. If the image of the original contains a regular or periodic pattern as in an already halftoned print, said regular or periodic pattern will interfere with the halftone dots in the output image in the subsequent halftoning process, thereby producing a moire pattern in the resulting image.
One solution to the above problem is to adjust the angle of a halftoning screen in such a way as to minimize the generation of a moire pattern. However, selecting an optimum screen angle is in itself a tedious and time-consuming task and requires skill on the part of the operator. Another proposal involves alteration of the aperture size of the halftone screen for blurring the image, so that any regular or periodic pattern contained in the image information will become less discernable. However, this method is disadvantageous in that the processed image is relatively unclear and hence poor in quality. In addition, the applicability of this method is considerably limited since it can be used only in the case where the aperture size is physically alterable.
Under these circumstances, the assignee previously proposed in U.S. patent application Ser. No. 543,525 (Jun. 26, 1990), now U.S. Pat. No. 5,065,255 a method and apparatus for processing image signals, in which the image information on an original is first processed for unsharpness by smoothing (averaging) and then processed for enhanced sharpness by sharpening so that a sharp image free from a moire pattern can be reproduced not only from original images having no regular or periodic pattern but also from those having regular or periodic patterns. The proposed method is characterized by a smoothing step in which the image information is processed for unsharpness by averaging with reference to a given constant mask size. Since the mask size is so selected that it corresponds to a comparatively fine screen, the method is reasonably effective in preventing the generation of a moire pattern if the original is of a common type which has a fairly small pitch of halftone dots. However, in such cases as where the halftone dots in the input original are coarse or where the smoothing step is preceded by exposure at an enlarged scale, the pitch of halftone dots in the input original is increased to become larger than the fixed mask size. If the mask size is smaller than the pitch of halftone dots in the input original, only insufficient smoothing is performed and the output halftone image contains a moire pattern that degrades its quality.
In short, the method proposed by U.S. patent application Ser. No. 543,525 is practically ineffective if the original is either a print that carries an image of coarse halftone dots or a print that has a fairly small pitch of halftone dots but which is enlarged by a large scale so that a regular pattern of a long period of variations is superposed on the original image signal to be read. In such cases, the noise level of the original image signal may be reduced by processing for unsharpness (smoothing) but it cannot be completely removed and residual noise such as the above-mentioned regular pattern will inevitably occur. If the subsequent step of sharpening (processing for enhanced sharpness) is performed by an increased degree with a view to enhancing image sharpness, the residual noise is also sharpened and the periodic pattern once smoothed out will reemerge to enhance the moire pattern produced by its interference with the halftone dots in the output image, thereby leading to deteriorated image quality.